1. Field of the Invention
The present invention relates to a charging member that charges a member to be charged such as a photosensitive member or dielectric, a charging device and a process cartridge that is detachably mountable onto a main body of an image forming apparatus such as an electrophotographic copying machine or a printer.
2. Description of Related Art
In an image forming apparatus of an electrophotographic process or an electrostatic recording process, in recent years, as a charging means for uniformly charging an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric or other members to be charged to a predetermined polarity and potential, a charging device of a contact charging system which allows an electrically conductive charging member (contact charging member) to which a voltage is applied to be abutted against the member to be charged to charge the member to be charged has been put in practical use since such a charging device has the advantages such that ozone is low and a power is low as compared with a corona charger that is of a non-contact type.
In particular, a contact charging device of a roller charging system which uses an electrically conductive elastic roller (hereinafter referred to as xe2x80x9ccharging rollerxe2x80x9d) as the contact charging member, brings the charging roller into pressure contact with the member to the charged, and applies a voltage to the member to be charged, thereby charging the member to be charged is preferably used from the viewpoint of the charging stabilization.
More specifically, charging starts by applying a voltage of a predetermined threshold voltage or higher because charging is conducted by discharging from the charging roller to the member to be charged.
For example, in the case where the charging roller is brought into pressure contact with an electrophotographic OPC photosensitive member which is 25 xcexcm in thickness as the member to be charged to conduct a charging process, when a voltage (threshold voltage) of about 600 V is applied to the charging roller, the surface potential of the photosensitive member starts to go up, and thereafter a photosensitive member surface potential increases linearly with a slope of a line with respect to an applied voltage being 1 (one). In the following description, that threshold value is defined as xe2x80x9ccharging start voltage Vthxe2x80x9d.
That is, in order to obtain a photosensitive member surface potential VD (dark section potential) required for image formation, it is necessary to apply a d.c. voltage of VD+Vth to the charging roller. A contact charging system in which only the d.c. voltage is applied to the contact charging member to charge the member to be charged is called xe2x80x9cd.c. charging systemxe2x80x9d.
There is also xe2x80x9ca.c. charging systemxe2x80x9d in which an oscillation voltage resulting from superimposing an a.c. voltage having a peak-to-peak voltage of 2xc3x97Vth or higher on the d.c. voltage corresponding to a desired surface potential VD of the member to be charged is applied to the contact charging member to charge the member to be charged as disclosed in Japanese Patent Application Laid-Open No. 63-149669.
The above a.c. charging system is proposed for the purpose of leveling the potential due to the a.c. voltage, and the surface potential of the photosensitive member which functions as the member to be charged converges on the intermediate point of a peak voltage as the photosensitive member gets further away from the contact charging member. The a.c. charging system has the disadvantages in that the high-voltage costs increase and the photosensitive member is damaged (scratched out).
The d.c. charging system is excellent in view of less increase in the high-voltage costs or less damage (scratching) of the photosensitive member but has the disadvantage in that the uniformity of the charging is insufficient as compared with the a.c. charging system. In an image forming apparatus of the transfer type that uses the contact charging device of the d.c. charging system as a photosensitive member charging process means, it is necessary to dispose a charge eliminating means (potential control means) such as a pre-exposure device in order to make uniform the surface potential of the photosensitive member which is nonuniform after the photosensitive member has passed through a transferring portion between the transferring portion and the charging portion of the photosensitive member.
In the case where a charge eliminating means such as a pre-exposure device is omitted, in low-temperature and low-humidity environments (temperature of 12 to 17xc2x0 C., humidity of 5 to 15%, hereinafter referred to as xe2x80x9cL/L environmentsxe2x80x9d)? when a halftone image is outputted in a state where the surface potential of the photosensitive member which has been disturbed by the transferring portion cannot be made uniform by the charging roller, a transversal streak unevenness remarkably occurs.
An object of the present invention is to provide a charging member, a charging device and a process cartridge capable of conducting uniform charging in a d.c. charging system.
Another object of the present invention is to provide a charging member, a charging device and a process cartridge, which are suitable for a simple structure having no charge eliminating means such as a pre-exposure device.
Still another object of the present invention is to provide a charging member, a charging device and a process cartridge, which are capable of conducting charging without unevenness when a halftone image is outputted under low-temperature and low-humidity environments.
Yet still another object of the present invention is to provide a charging member, a charging device and a process cartridge, which do not produce transversal streak unevenness.